The present invention relates to chip carrier sockets used to attach the leads extending from a chip carrier to conductors on a substrate. The primary advantage to using these sockets is that they allow for removal of the chip carrier without having to undertake the tedious disconnection and reconnection tasks associated with soldered surface mount connections. Use of these sockets is then particularly appropriate where the device is expected to be upgraded, prone to failure or at the prototype level where it may be necessary to remove a chip carrier for testing and debugging purposes.
In U.S. Pat. No. 5,007,845, a low height socket is disclosed (see FIG. 1) that uses a tuning fork shaped contact 11 within a socket body 1 and a cover 19 that acts to wedge the leads 13 of the chip carrier 3 into electrical engagement with the contacts 11. By removing the cover 19, the chip carrier 3 can be easily replaced. As shown in FIG. 1, the socket body 1 is of monolithic molded plastic construction and is specifically designed to accommodate the particular chip carrier 3. The socket body 1 has a bottom 5 and four upstanding walls 7 extending therefrom. A plurality of slots 9 are disposed in the walls for receiving contacts 11 that correspond to the leads 13 of the chip carrier 3 to be electrically connected to the conductors 15 on the substrate 17. These socket bodies 1 require an intricate mold that requires a significant amount of time to produce due to the complex and small features that must be incorporated which results in a high initial expense.
An advantage of a chip carrier socket is that the electrical connection between the chip carrier leads and the conductors on the substrate is more easily interchanged than when the leads are directly connected to the conductors, such as by soldering. A particular contact and socket assembly that facilitates easy assembly and disassembly is described in U.S. patent application Ser. No. 07/969,597, which was filed Oct. 30, 1992 is hereby incorporated by reference as though set forth verbatim herein.
The invention of the referenced application includes a tool receiving recess within the contacts so that support may be provided to prevent the forces associated with inserting the cover from being transferred to the contact/conductor connection. This tool receiving recess enhances the usefulness of the sockets in the product development stage of substrate design where it may be necessary to remove and replace the chip carrier a number of times by protecting the electrical connection between the contact and the conductor.
As more and more complicated chip configurations are developed the number of leads extending from the chip carrier and the overall size of the chip carrier are apt to change. Furthermore, it is likely that chip carriers will be designed specifically for a particular application. Socket assemblies are particularly useful with these specialized chip carriers due to their ability to maintain electrical engagement with the conductors while the chip carrier is being installed and removed a number of times. It is likely that many specialized applications will include large chip carriers, such as 4 inches by 4 inches. In these uses, it is especially important to maintain the coefficients of thermal expansion between the socket body and the substrate essentially identical in order to maintain alignment of the contacts and the conductors with minimal stress being imposed on the interface.
However, it typically is not economically feasible during the product development stage to justify the expense or lead time associated with chip carrier specific molded socket bodies for non-standard chip carrier configurations. U.S. Pat. No. 5,062,802 addresses this problem by creating a socket-like seat from a number of contact modules containing contacts electrically connect the chip carrier leads to the conductors on the substrate. A removable plate is used to orientate the modules so that the contacts housed therein correspond to the conductors on the substrate and the expected position of the leads. Once the contacts are engaged with the conductors on the substrate the plate is removed.
A problem with the '802 invention is that it is not suitable for applications involving quantities above initial prototype applications. The construction techniques are awkward and time intensive. The '802 invention does not provide a user-friendly socket body because a distinct chip carrier socket body is not really created. Therefore, it is not possible to use the invention with the same board loading equipment that would be used with a chip carrier specific molded socket body.
Therefore, to enable chip carrier sockets to reach their presently envisioned potential what is needed is a chip carrier socket for non-standard chip carriers that is economically viable at low levels of production and has the characteristics of a chip carrier specific molded socket body without the associated high tooling costs and long lead times. The socket would be useful in product development where the advantages of socket assembly of chip carrier to substrate are most applicable. Chip carrier sockets would also be usable in unique applications where, due to the quantities involved, it may never be feasible to utilize a chip carrier specific molded socket body. Finally, because the socket would have the characteristics of a monolithic molded socket body, the socket body would enable production of substrates that are going to use a chip carrier specific molded socket body while the molded body is being developed, thereby enabling the set up of production board loading equipment without the delay of waiting for the molded components.